Magnetic recording medium and method of fabricating the same

ABSTRACT

A read-only disk and a write-only disk recorded at high density having a magneto-optical film differing in coercive force depending on the information to be recorded disposed on a substrate. In the read-only and write-once disks, signals can be read employing only part of the irradiation beam, and thus super-resolution reading is realized.

This application is a division of U.S. application Ser. No. 08/343,203filed Nov. 28, 1994 now U.S. Pat. No. 5,993,937.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium used inrecording of a information. More particularly it is used in an externalmemory of computer, video or audio recording apparatus, game machine orother memory device, or integrated multi-media.

2. Description of the Prior Art

Recently, read-only recording media are widely spread for audio, videoand data files, such as CD, LD, and CD-ROM. However, along with thedistribution of optical recording media, lately, as versatile data arehandled, there is a growing demand for higher density.

As one of the solutions, in the rewritable magneto-optical recordingmedia, it is proposed to read a tiny written mark smaller than a laserspot diameter by the following method. That is, in the constitutionhaving a separate recording layer and a reading layer, only in a limitednarrow region in part of an irradiation region of an optical beam forreading, the information recorded in the recording layer is read whileforming a state copying into the reading layer, and this method iscalled super-resolution reading.

This is the method disclosed in the Japanese Laid-open Patent No.03-93058, and basically it is composed of two perpendicular magnetizedfilms comprising a recording layer of high coercive force and a readinglayer of low coercive force, and an exchange coupling force acts betweenthe magnetized films. The reading principle of the thus composedmagneto-magneto-optical recording medium is briefly described below. Atroom temperature, only the magnetization of the reading layer is alignedin one direction by a powerful external magnetic field (3 Koe or more).However, in the region elevated in temperature as the optical beam forreading is irradiated, the coercive force of the reading layer dropsabruptly, and the written mark of the recording layer is copied in thereading layer by the exchange coupling action. Therefore, signals aregenerated only from the portion heated by the irradiation of opticalbeam for reading, whereas no signal is generated from the portion notelevated in temperature in the region irradiated with optical beam forreading.

This method is effective means for the recording medium to be recordedby the user as in the conventional magneto-optical recording, but is notapplied to the read-only magneto-optical recording medium. As the methodof super-resolution reading on a read-only magneto-optical recordingmedium, a method of a making use of difference between the reflectanceof solid-phase state and reflectance in liquid-phase state is known(Yasuda et al. International Symposium Optical Memory and Optical DataStorage '93 Th3.2, 1993). In this method, GeSbTe is disposed on asubstrate in which information is recorded in pit form, and areflectance layer is further laminated. The principle of reading of themagneto-optical recording medium in this constitution is brieflydescribed below. When reading by irradiating an optical beam from thesubstrate side, at room temperature, GeSbTe is in a solid-phase state,and the light transmittance of GeSbTe is large, and hence a certainreflected light is obtained. However, when the temperature is elevatedby the optical beam for reading, GeSbTe is set in a liquid-phase state,and the light transmittance of GeSbTe decreases significantly, and thereflected light becomes extremely small.

Therefore, the read signal is limited to a cold portion only of theregion irradiated with an optical beam for reading, and super-resolutionreading is enabled in the read-only magneto-optical recording medium.

In such constitution, however, there are the following defects. Thefirst problem is that an extremely large power is required in spite ofread-only use because the reading temperature must be more than severalhundred degrees in order to read while heating always to theliquid-phase state of GeSbTe. It means not only that a large outputlaser is needed, but also that substrate deterioration due to repeatedreading or deterioration of GeSbTe film may result.

The second problem is that it is likely to pick up the crosstalk of theadjacent track to make it hard to narrow the track pitch because onlythe cold portion is read while masking the hot portion of the regionirradiated with optical beam.

The third problem is that it is hard to use part of the recording mediumfor reading only and the other as a rewritable recording medium, thatis, so-called partial ROM, or rewritable data file having read-onlyaddress information or control information. That is, the GeSbTe filmitself is a rewritable recording medium making use of reversible changesbetween crystal state and amorphous state, but if the information isrecorded by keeping this film always in a liquid-phase state, it is readwhile erasing it, and the phase-change recorded information cannot beread again.

SUMMARY OF THE INVENTION

In the light of the above problems, it is hence a primary object of theinvention to present a magneto-optical recording medium for readingonly, capable of performing super-resolution at a low operatingtemperature, narrowing the track pitch, and disposing on a same plane asa rewritable recording medium.

To solve the problems, in one magnetic recording medium of theinvention, at least a partial region on the substrate is composed of aportion differing in roughness of surface, corresponding to the recordedinformation, and the magnetic characteristic of the magnetic film formedon the substrate is varied in each portion. Herein, one of the portionsdiffering in roughness of surface is called a roughness portion, and theother is called a flat portion. The written mark corresponding to therecorded information may be formed either in the roughness portion or inthe flat portion.

In the invention, the magnetic characteristic of the magnetic filmprovided on the substrate is different between the roughness portion andthe flat portion. More specifically, it is desired that the coerciveforce at room temperature is different on the basis of the difference inthe magnetic characteristic between the roughness portion and flatportion. However, the invention is not limited to this alone, and if thecoercive force is the same at room temperature, the coercive force maybe different at a temperature higher than room temperature. Moreover, onthe basis of a difference in the magnetic characteristic, at roomtemperature or a temperature higher than room temperature, thedifference may be such that it is a perpendicular magnetized film in theroughness portion and an in-plane magnetized film in the flat portion.

Incidentally, for reading these magnetic recording media, a conventionalmagnetic head or magneto-optical head is used. As the substrate of themagnetic recording medium of the invention, various materials may beused, including an inorganic oxide, polymer resin, and metals. Aboveall, polymer resins such as polycarbonate, olefin and PMMA arepreferable because the read-only information composed of differentroughness portions can be easily mass-duplicated by injection.

The magnetic film is not particularly specified as far as the magneticcharacteristic such as coercive force and magnetic anisotropy varies dueto difference in roughness of surface. In particular, when reproducingwith a magneto-optical head, an alloy of rare earth and transition metalcomposed of at least one rare earth selected from Gd, Td, Dy, and Nd,and at least one transition metal selected from Fe and Co is preferable.Of course, a magnetic film composed of them together with additiveelements such as Cr and Pt is also preferable.

In other magnetic recording medium of the invention, at least in apartial region, a micro-structure (derived from fluctuation ofcomposition, density, crystallinity, etc.) that can be observed bytransmission electron microscope in a magnetic film, or an atomicordering structure, or a portion differing in crystal grain size isformed, corresponding to the recorded information, so that the magneticanisotropy of the magnetic film may differ in each portion.

Moreover, the recording method into the magneto-optical recording mediumof the invention comprises, when recording the information,

1) a constitution for varying the magnetic characteristic such ascoercive force and magnetic anisotropy of the magnetic film in eachportion, corresponding to the recorded information, by recording themagnetic characteristic in the magnetic film at such light intensity asto change irreversibly;

2) a constitution for varying the coercive force of the magnetic film ineach portion, by forming portions differing in the magnitude of magneticcharacteristic, corresponding to the recorded information, by recordingat such light intensity as to diffuse mutually the laminated magneticfilm and additive element film; or

3) a constitution for varying the coercive force of the magnetic film ineach portion, by forming portions different in the micro-structure orcrystal grain size, corresponding to the recorded information, byrecording at such light intensity as to grow the micro-structure orcrystal grain size that can be observed by a transmission electronmicroscope in the magnetic film.

These constitutions make use of the irreversible change by light beam,and are suitable for composing a write-once recording medium.

In such constitutions, the invention presents a read-only magneticrecording medium or a write-once magnetic recording medium of high S/Nratio, capable of reading at super-resolution at low operatingtemperature, and narrowing the track pitch, and it can be disposed onthe same plane as a rewritable recording medium.

Its principle is more specifically described below.

First, the means for changing the coercive force of the magnetic film isdescribed.

If there is a micro-structure or crystal boundary derived fromfluctuation in composition, density, crystallinity or the like that canbe observed by a transmission electron microscope in a magnetic film,the homogeneity in the film is impaired in that portion, and adisturbance occurs in the domain wall energy. Herein, the domain wallenergy is an energy reserved in the domain wall, and when the domainwall energy varies with the position, in order to move the domain wall,a force expressed by its integral value is required. On the other hand,the domain wall is a region in which the direction of the spin which isthe source of magnetization changes gradually, and it has a specificwidth, and therefore when the size of the microstructure or crystalgrain becomes smaller than the width of the domain wall, theheterogeneity derived from them is averaged within the domain wall, andtherefore the interaction of heterogeneity and domain wall becomessmaller, and it is easy to move the domain wall.

In the crystalline magnetic film, meanwhile, when the crystallinemagnetic anisotropy is large, magnetization in each crystal grain islikely to occur in the direction of each easy axis for magnetization. Inan ordinary polycrystalline film, since the direction of the crystalplane of each crystal grain (that is, the direction of easy axis formagnetization) is distributed randomly, if it is attempted to magnetizethe magnetic film in one direction, the magnetization in the directiondiffering in each crystal grain is aligned in one direction whilerotating. At this time, when the size of the crystal grain issufficiently larger than the width of the domain wall, the domain wallis likely to exist in the boundary of each crystal grain, and movementof the domain wall is impeded, while, when the crystal grain size issmaller than the width of the domain wall, the domain wall is presentover plural crystal grains, and the crystalline magnetic anisotropy ofeach crystal grain is averaged within the domain wall, and hence theinteraction with the domain wall decreases, so that it is easier tomagnetize.

That is, the coercive force can be changed by a specific magnituderelation of the domain wall width with the size of micro-structure orcrystal grain.

Incidentally, when there is a certain roughness of surface on thesubstrate, the magnetic film formed thereon by sputtering method orvapor deposition method must be grown with a directivity in thedirection of each micro-structure for composing the roughness surface.The micro-structures for composing the roughness surface have variousdirectivities, and the magnetic film formed on the roughness surface isheterogeneous in density and crystallinity near the boundary of themicro-structures. Or when a crystalline magnetic film having a largecrystalline magnetic anisotropy is formed on a roughness surface,directions of crystal grains (that is, directions of the easy axis formagnetization) are not uniform.

Therefore, if there is a certain roughness of surface on the substrate,the magnetic film formed thereon has a micro-structure or crystal grainboundary derived from fluctuation of density and crystallinityreflecting such roughness of surface.

If such micro-structure or crystal grain boundary is present, asmentioned above, it is possible to vary the coercive force by therelation of a certain magnitude between the domain wall width and sizeof micro-structure or crystal grain.

However, if the size of the micro-structure or crystal grain is morethan a specific value, it is detected as noise at the time of opticalreading, and the reading S/N ratio is lowered. Therefore, the size ofmicro-structure or crystal grain should be defined within a certaindimension so as not to cause noise.

Furthermore, when a specific element is added to the magnetic film, themagnetic anisotropy of the magnetic film may be increased or decreased.For example, such phenomenon is witnessed when a heavy rare earth metalelement or other 3d transition metal element is added to a rareearth-transition metal compound magnetic film or 3d transition metalcompound magnetic film. Generally, the magnitude of coercive force andmagnitude of magnetic anisotropy are in positive relation, and in nearlysimilar materials, when the magnetic anisotropy is large, the coerciveforce is also large, or when the magnetic anisotropy is small, thecoercive force is also small. Hence, in the magnetic film used asrecording film, if there are portions differing in magnetic anisotropydue to composition changes, the coercive force can be varied inindividual portions.

The principle of reading of the recording medium of the invention isdescribed below. In a magneto-optical recording medium having differentcoercive forces depending on the information to be recorded,

1) the entire medium is magnetized in a magnetic field greater than anycoercive force, and then

2) it is magnetized again reversely in a magnetic field greater than onecoercive force and smaller than other coercive force, so that a writtenmark (domain) depending on the information to be recorded can be easilyformed without recording in drive. Thus the prepared magnetic recordingmedium can be easily read by using an ordinary magnetic readingapparatus or magneto-optical reading apparatus, same as the conventionalwritten mark recorded magnetically. This method is particularlyconvenient when presenting a read-only medium corresponding to anexisting commercial magneto-optical drive.

The reading principle of super-resolution of recording medium of theinvention is described below.

In FIG. 1(a), reference numeral 11 is a magnetic film composed of amagneto-optical film, comprising a low coercive force portion 11a and ahigh coercive force portion 11b corresponding to the recordedinformation. Reference numeral 12 is a reading light spot, 13 is amagnetic field for initializing, and 14 is a bias magnetic field. Curve15 is a curve for showing the temperature of the magneto-optical filmwhen reading, and corresponds to the position of the reading light spot12. FIG. 1(b), curve 16 represents the temperature characteristic ofcoercive force of the high coercive force portion 11b, while curve 17shows the temperature characteristic of the coercive force of the lowcoercive force portion 11a.

When reading, prior to irradiation with light for reading, themagneto-optical film is uniformly magnetized upward (or downward) in theinitializing magnetic field 13. At this time, the intensity of magneticfield H2 of the initializing magnetic field 13 is required to be morethan the coercive force H3 of the high coercive force portion 11b atroom temperature. When the recording medium moves and passes through thereading light spot 12, the temperature of magneto-optical film varies asindicated by curve 15. At this time, a weak bias magnetic field 14 in areverse direction of the initializing magnetic field 13 is applied, andthe intensity of its magnetic field is H1.

Along with passing of the reading light spot 12, the temperature of themagneto-optical film rises, but until reaching temperature T1, the lowcoercive force portion 11a and high coercive force portion 11b bothmaintain a large coercive force by the bias magnetic field 14, so thatthe direction of magnetization is not changed. However, in thetemperature range higher than T1 and lower than T2, in the low coerciveforce portion 11a indicated by curve 17, the bias magnetic fieldintensity H1 is superior to the coercive force, and the magnetization isinverted along the direction of the bias magnetic field 14. On the otherhand, in the high coercive force portion 11b, since the coercive forceindicated by curve 16 is superior to the bias magnetic field intensityH1, the magnetization is not inverted, and remains in the directiondetermined by the initializing magnetic field 13. As the temperaturerises further, in the higher temperature region than T2, even in thehigh coercive force portion 11b indicated by curve 16, the bias magneticfield intensity H1 is superior to the coercive force, and themagnetization is inverted along the direction of the bias magnetic field14. That is, of the reading light spot 12, only region A higher intemperature than T1 and lower than T2, the state of invertedmagnetization is created along the recorded information, therebycontributing to detection and reading of recorded information. That is,the recording domain smaller than the reading light spot diameter hardto detect in the ordinary reading method can be detected and read.

In the above explanation of reading method, the intensity of light forreading was such that the maximum temperature of the magneto-opticalfilm be T2 or more, but when the intensity of light for reading is suchthat the maximum temperature be somewhere between T1 and T2, themagnetization is inverted along the recorded information only in theregion of the temperature of T1 or more and T2 or less, out of thereading light spot 12, so that reading operation of super-resolution maybe realized.

The magneto-optical recording medium for realizing such super-resolutionreading may be composed of any magnetic film differing in the coerciveforce depending on the recorded information and possessing a relativelylarge magneto-optical effect.

As the reading method for realizing such super-resolution reading byusing the magneto-optical recording medium of the invention, readingwhile raising the temperature of the magneto-optical film over T1, andbias magnetic field 14 are needed, but the initializing magnetic field13 is not necessarily required. That is, in the principle diagram inFIG. 1(a), the initializing magnetic field is provided in order to alignthe magnetization upward before reading action, but in the case ofreading while raising the temperature of the magneto-optical film, owingto the downward bias magnetic field H1, the magnetization is uniformlydownward after passing through the reading light spot 12, and by settingthe bias magnetic field H1 upward when reading next time, theinitializing magnetic field can be omitted.

These reading methods are convenient for keeping compatibility with theexisting super-resolution reading methods.

Moreover, a recording method into some of the optical recording media ofthe invention is described below.

Generally, in the magnetic film as it is formed on a substrate bysputtering method or the like, the micro-structure or crystal grainderived from fluctuation of density or crystallinity is likely to befine, and this tendency is strong when deposited at low temperature.Such state is metastable as the energy state of substance, and byheating, the micro-structure or crystal grain grows in a form ofcoupling adjoining micro-structures or crystal grains mutually so as tomigrate to stabler state.

Accordingly, using the magnetic film in the deposition state with thesize of the micro-structure or crystal grain smaller than the width ofthe domain wall of the magnetic film as the recording film, by recordingat a light intensity capable of growing the size of the micro-structureor crystal grain in the recording film when recording the information, acoarse portion of micro-structure or crystal grain can be formedcorresponding to the recorded information. When there are different sizeportions in the micro-structures or crystal grains, as mentioned above,the coercive force can be varied by the specific magnitude relation ofthe width of domain wall and micro-structure or crystal grain.

Incidentally, the coarse portion of micro-structure or crystal grainformed by this recording method can be formed finely by heating over themelting point of the magnetic film and quenching, and therefore therecorded information can be erased by the difference in magnitude of themicro-structure.

Besides, in the magneto-optical recording medium of which recording filmis a laminate film composed of a magnetic film having a certain magneticanisotropy and a film made of an additive element for increasing ordecreasing the magnetic anisotropy of the magnetic film, in that state,there is no portion evidently different in the magnetic anisotropy inthe magnetic film. However, when the magnetic film is heated over acertain temperature, the laminated magnetic film and additive elementfilm begin to diffuse mutually, and are being alloyed gradually. In thealloyed portion, the magnetic anisotropy of the magnetic film increasesor decreases, so that the coercive force also increases or decreases.

Hence, at the time of information recording, by recording at theintensity of light capable of mutually diffusing the laminated magneticfilm and additive element film, portions differing in the magnitude ofmagnetic anisotropy are formed, and the coercive force of the magneticfilm may differ in individual portions.

So far are described the recording method and reading method of magneticrecording medium having different coercive forces depending on theinformation to be recorded. It is, however, not the only intention ofthe invention to provide coercive force differences depending on theinformation to be recorded. For example, depending on the information tobe recorded, one may be formed in a perpendicular magnetized film, andthe other in an in-plane magnetized film. These can be realized easilyby making use of the phenomenon that the perpendicular anisotropy islost when irradiated with a further stronger power although the coerciveforce is lowered when rare earth-transition metal material is irradiatedwith a stronger power than in ordinary recording. That is, whenirradiated with a stronger power than in recording, the atomic orderingstructure is relaxed, and the perpendicular anisotropy decreases, andhence the coercive force is lowered. Therefore, when irradiated with afurther stronger power, the perpendicular anisotropy is furtherdecreased to form an in-plane magnetized film.

On the other hand, at room temperature, if it is an in-plane magnetizedfilm (coercive force zero) regardless of the information to be recorded,when elevated in temperature by irradiation with optical beam forreading, it becomes a perpendicular magnetized film, and hence theinvention is effective also in the magneto-optical film possessing adifferent coercive force depending on the information to be recorded atthis time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are principle drawings for explaining the operationof the invention.

FIG. 2 is a constituent diagram of a magneto-optical recording medium ina first embodiment of the invention.

FIGS. 3(a) and 3(b), respectively, show and a sectional view of asubstrate in the magneto-optical recording medium in the firstembodiment of the invention.

FIGS. 4(a) and 4(b) are diagrams showing a fabricating procedure of thesubstrate in the magneto-optical recording medium in the firstembodiment of the invention.

FIG. 5 is a diagram showing the relation of the in-plane directiondimension of roughness of surface and coercive force of magnetic film ofthe substrate in the magneto-optical recording medium in the firstembodiment of the invention.

FIG. 6 is a diagram showing the relation of the perpendicular directiondimension of roughness of surface and coercive force of magnetic film ofthe substrate in the magneto-optical recording medium in the firstembodiment of the invention.

FIG. 7 is a diagram showing the temperature dependence of coercive forceof the magnetic film in the portion differing in the roughness ofsurface of the substrate, in the magneto-optical recording medium in thefirst embodiment of the invention.

FIG. 8 is a diagram showing the relation between the in-plane directiondimension of roughness of surface and reading noise of the substrate inthe magneto-optical recording medium in the first embodiment of theinvention.

FIG. 9 is a diagram showing the relation between the recording domaindimension and the signal level, in the case of super-resolution readingof the magneto-optical recording medium in the first embodiment of theinvention.

FIG. 10 is a diagram showing the relation of the in-plane directiondimension of roughness of surface and coercive force of magnetic film ofthe substrate in a magneto-optical recording medium in a secondembodiment of the invention.

FIG. 11 is a diagram showing the relation of the perpendicular directiondimension of roughness of surface and coercive force of magnetic film ofthe substrate in the magneto-optical recording medium in the secondembodiment of the invention.

FIGS. 12(a) and 12(b) are constituent diagrams of a recording medium ina third embodiment of the invention.

FIG. 13 is a perspective view of a basic portion of a reading apparatusin the embodiment of the invention.

FIG. 14 is a constituent diagram of a magneto-optical recording mediumin a fourth embodiment of the invention.

FIGS. 15(a) and 15(b) are schematic diagrams of a recording apparatusfor the magneto-optical recording medium in the fourth embodiment of theinvention.

FIG. 16 is a diagram showing the relation of Tb content and coerciveforce in GdTbFeCo film.

FIG. 17 is a diagram showing the relation of Fe content and coerciveforce in GdTbFeCo film.

FIG. 18 is a constituent diagram of a magneto-optical recording mediumin a fifth embodiment of the invention.

FIG. 19 is a principle diagram for explaining the operation in a sixthembodiment of the invention.

FIG. 20 is a Kerr hysteresis loop of a first magnetic layer in the sixthembodiment of the invention.

FIGS. 21(a) and 21(b) are principle diagrams for explaining theoperation of a seventh embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, the magnetic recording medium of theinvention is described below in some of its embodiments.

First embodiment

FIG. 2 is a sectional view showing a constitution of a magneto-opticalrecording medium in a first embodiment of the invention, and FIG. 3shows a perspective view and a sectional view of the substrate in themagneto-optical recording medium of the embodiment.

In FIG. 2, a first protection layer 22, a magnetic film 23, a secondprotection layer 24, and a reflectance layer 25 are sequentially formedon a substrate 21 by RF sputtering method and DC sputtering method, anda protection coat layer 26 is further formed by spin coating method.Herein, showing examples of materials of constituent elements, thesubstrate 21 is made of polycarbonate or transparent plastic or glass,the first protection layer 22 and second protection layer 24 are made ofnitride film such as SiN, the magnetic film 23 is a rareearth-transition metal compound magnetic film such as GdTbFeCo film, thereflectance layer 25 is a metal film such as Al film, and the protectioncoat layer 26 is an acrylic UV resin. Examples of thickness of the filmsare 100 nm for the first protection layer 22, 15 to 40 nm for themagnetic film 23, 10 to 20 nm for the second protection layer 24, 40 nmfor the reflectance layer, and 5 μm for the protection coat layer 26.

The substrate 21 is magnified and schematically shown in FIG. 3. InFIGS. 3(a) (a), and 3(b), the shaded area refers to a recording domain31 formed on the surface of the substrate corresponding to the recordinginformation, and its A-A' section is shown in FIG. 3(b). In themagneto-optical recording medium of the embodiment, by roughening therecording domain portion 31, on the substrate, as compared with theother portion, the recording information is expressed. Undulated groovescomprising 32 and 33 are guides for tracking control of the light spotat the time of reading.

Such substrate 21 can be fabricated easily by mass duplication method ofconventional substrates, such as injection method, by using amaster-substrate forming roughness portion and a flat portion (smoothsurface), corresponding to the recording information. The fabricatingprocedure is described in two methods below while referring to thedrawings.

A first method is explained. In this method, as in this embodiment, byroughening the recording domain area as compared with other area, therecording information is expressed. In FIG. 4(a), reference numeral 41denotes a glass master-substrate, 42 is a photo resist, 43 is an argonlaser beam, 44 is an etching ion particle beam, 45 is an ultravioletray, 46 is an Ni film, 47 is an Ni electroforming layer, 48 is astamper, and 21 is a substrate.

The forming procedure is the sequence of 1), 2), 3) and so forth in FIG.4(a).

1) The photo resist 42 applied on the smooth polished glassmaster-substrate 41 is heated, and exposed by cutting by the argon laserbeam 43 modulated in power depending on the information to be recorded.

2) By developing in a developing solution, the photo resist 42 removedportions and unremoved portions are formed depending on the informationto be recorded. So far, the procedure is same as in the conventionalmanner.

3) In this embodiment, plasma ion etching is executed next. This step isintended to roughen the exposed portion of the glass master-substrate 21by developing. In this embodiment, using a mixed gas of CF₄ gas and Negas or a mixed gas of CF₄ gas and Ar gas, etching was performed for 20to 120 seconds at a pressure of 1 to 20 mTorr and power of 0.7 to 2.0W/cm². Herein, He gas is mixed in order to decrease the noise caused byroughening of the surface of the glass master-substrate. For example, bymixing a half amount Ne gas to CF₄ gas and etching, the noise level wasreduced by about 2 dB.

4) After completion of plasma ion etching, the exposed portion of theglass master-substrate 41 and the surface of photo resist 42 wasroughened.

5) Furthermore, the entire surface was irradiated with ultraviolet ray45, and

6) Developed again in a developing solution, so that the photo resist 42formed on the glass master-substrate 41 was completely removed.

7) After deposition of Ni film 46 on the glass master-substrate 41 bysputtering, it was used as an electrode, and a nickel electroforminglayer 47 was provided in a specific thickness, and a stamper 48 wasformed.

8) Peeling the stamper 48 off the glass master-substrate 41, the stamper48 forming a roughness portion and a flat portion depending on theinformation to be recorded was completed. In this case, the recordingdomain area was the roughness portion.

9) Using this stamper 48, a substrate 21 corresponding to the roughnessof surface of the stamper 48 was fabricated by injection method or thelike.

If the substrate material is glass, meanwhile, and copying by injectionis difficult, the substrate may be fabricated directly in the stepsof 1) to 6) assuming the glass substrate to be a glass master-substrate.

A second method is explained. In this method, different from the aboveembodiment, the recording information is expressed by smoothing therecording domain portion as compared with the other area (roughening theother portion than the recording domain). In FIG. 4(b), referencenumerals indicate the same as in FIG. 4(a), except that the photo resist42 is a material which has a property of being softened when sensitizedto argon laser or softened in a high temperature state by argon laserirradiation. The fabricating procedure is the sequence of 1), 2), 3) andso forth in FIG. 4(b).

1) A photo resist 42 is applied on a smoothly polished glassmaster-substrate 41, and is heated. So far, the procedure is exactlysame as in the conventional procedure.

2) In this embodiment, plasma ion etching is effected next. This step isintended to roughen the entire surface of photo resist 42. In thisembodiment, using a mixed gas of N₂ gas and He gas or a mixed gas of O₂gas and He gas, etching was performed for 20 to 120 seconds at apressure of 1 to 20 mTorr and power of 0.2 to 0.5 W/cm². Herein, He gasis mixed in order to decrease the noise generated by roughening thephoto resist surface. For example, by mixing a half amount of He to N₂gas and etching, the noise level was reduced by about 2 dB.

3) The surface of the photo resist 42 was roughened after completion ofplasma ion etching.

4) On the roughened photo resist 42, argon laser beam 43 modulated inpower depending on the information to be recorded is emitted to softenand smooth the roughened surface of the photo resist. Therefore,depending on the information to be recorded, the flat portion androughness portion are formed on the surface of the photo resist 42.

5) After depositing Ni film 46 on the photo resist 42 by sputtering,using it as an electrode, a nickel electroforming layer 47 is providedin a specific thickness, and a stamper 48 is formed.

6) Peeling off the stamper 48 from the glass master-substrate 41 andphoto resist 42, the stamper 48 forming a flat portion and a roughnessportion depending on the information to be recorded is completed.

7) Using this stamper 48, a substrate 21 corresponding to the roughnessof surface of the stamper 48 is fabricated by injection method or thelike.

Back to FIGS. 3(a) and 3(b) the roughness of surface in the recordingdomain area 31 is expressed by dimension α in the in-plane direction anddimension β in the perpendicular direction, and each mean in therecording domain area 31 is expressed by <α> and <β>. FIG. 5 shows therelation between the coercive force and <α> of the magnetic film(GdTbFeCo film) in the case of <β> up to 5 nm, and FIG. 6 shows therelation between the coercive force and <β> in the case of <α> up to 45nm.

It is known from FIG. 5 that the coercive force begins to increase whenthe mean dimension <α> of the roughness of surface in the in-planedirection becomes about 10 nm or more, and it is also known from FIG. 6that the coercive force begins to increase when the mean dimension <β>of the roughness of surface in the perpendicular direction becomes about3 nm or more.

Incidentally, seeing the magnetic anisotropy energy Ku of GdTbFeCo filmof up to 2×10⁸ erg/cc, and the exchange stiffness constant A of up to2×10⁻⁷ erg/cm, the domain wall width δ=π (A/Ku)^(1/2) is about 10 nm.

This fact corresponds very well with the phenomenon that the coerciveforce begins to increase when the mean dimension <α> of roughness ofsurface in the in-plane direction becomes about 10 nm or more in FIG. 5.This result proves the action of the invention stated above that thecoercive force varies depending on a specific magnitude relation of thedomain wall width and micro-structure when the micro-structurereflecting the roughness of surface of the substrate exists. Besides,the width δ of the domain wall of a rare earth-transition metal compoundmagnetic film such as TbFe film and GdTbFe film is estimated to bearound 10 to 20 nm, and a corresponding relation between coercive forceand roughness dimension was recognized the same as in the GdTbFeCo film.

FIG. 7 shows the temperature dependence of the coercive force in theindividual portions of the magneto-optical recording medium of theembodiment, forming the roughness portion with <α> up to 40 nm and <β>up to 5 nm and the flat portion with <α> up to 80 nm and <β> up to 1 nmon the substrate. Curve 71 indicates the roughness portion, and curve 72denotes the flat portion. Herein, the curie temperature of the GdTbFeCofilm as magnetic film is about 210° C. in this embodiment, and acoercive force difference of about two times is obtained in the entiretemperature region from room temperature to curie temperature.

Hence, depending on the recording information, when the rareearth-transition metal alloy magnetic film is formed on the substratehaving the portion with the mean roughness of surface in the in-planedirection and perpendicular direction of 10 nm or more and 3 nm or morerespectively, and the portion with the mean roughness of surface in thein-plane direction and perpendicular direction of 10 nm or less and 3 nmor less, a magneto-optical recording medium possessing portionsdiffering in the coercive force depending on the recording informationis obtained.

However, in order to increase the coercive force, if the mean roughnessof surface in the in-plane direction and perpendicular dimension isrespectively 10 nm or more and 3 nm or more, it is not enough for themagneto-optical recording medium of the embodiment. That is, it isnecessary to prevent generation of noise in reading due to roughening.FIG. 8 shows the relation between the mean roughness in the in-planedirection <α> and reading noise at <β>=5 nm with laser of wavelength of830 nm, in which the reading noise increases suddenly when <α> exceedsabout 50 nm. Furthermore, at <α> up to 45 nm, if the mean roughness ofsurface in the perpendicular direction <β> becomes 20 nm or more, thereading noise increases, which is not appropriate.

Hence, the mean roughness of surface in the in-plane direction andperpendicular direction is preferred to be 50 nm or less and 20 nm orless, respectively.

Next, forming a magneto-optical recording medium setting the minimumdimension of the portions differing in the coercive force depending onthe recording information (that is, recording domain) at 1.0 μm, it waspreliminarily magnetized in batch in one direction, and a magnetic fieldin a reverse direction was applied in the magnetic field in theintermediate value of two coercive forces, and when it was read in aconventional magneto-optical reading apparatus, the signal could beread. Besides, in the same recording medium, by utilizing the portionwithout the coercive force differing portion corresponding to therecording information, when recorded (curie point recording) by using aconventional recording apparatus, recording and reading were effectedsame as in the prior art.

These facts mean that it is convenient for realizing a read-only disk ora partial ROM disk comprising a read-only portion and a rewritableportion suited to an existing commercial magneto-optical drive.

Furthermore, a magneto-optical recording medium setting the minimumdimension of the portions differing in the coercive force depending onthe recording information (that is, recording domain) at 0.6 μm or lesswhich is difficult to detect with a light spot with wavelength of 830 nmis read in a reading method mentioned in the Summary of the Invention. Amagneto-optical reading apparatus used in reading is shown in FIG. 13.The linear velocity is 5 m/s. What is particularly characteristic of theinvention in reading the magneto-optical recording medium is magneticfield applying means 132, while the other constitution is same as theordinary magneto-optical reading apparatus hitherto proposed. By themagnetic field applying means 132, as shown in FIG. 13, an initializingmagnetic field 13 from the S pole and a bias magnetic field 14 from theN pole are applied to a magneto-optical film 11, and the bias magneticfield 14 from the N pole is applied to the position of the optical beamfor reading. Each magnetic field may be always constant in direction andintensity, and therefore the magnetic field applying means 132 ispreferred to be a permanent magnet from the viewpoint of saving of powerconsumption and reduction of size. Moreover, by forming in a size enoughfor covering the recording region in the radial direction of the disk,it is more effective because it is not necessary to move the magneticfield applying means 132.

Defining the initializing magnetic field 13 in FIG. 10 to be 3 kOe andthe bias magnetic field 14 to be 500 Oe, in the condition of linearvelocity of 5 m/s, the magneto-optical recording medium setting therecording domain in various dimensions was read, and CN ratio changes(curve 91) at this time are shown in FIG. 9. For reference, CN ratiochanges (curve 92) in ordinary recording and reading are also shown. TheCN ratio on the axis of ordinates is plotted with the CN ratio at arecording domain dimension of 2 micron at 0 dB. As clear from FIG. 9, agreater CN ratio can be preserved for a smaller recording domaindimension, so that a magneto-optical recording medium of high density isrealized.

In addition, as known from FIG. 7, the temperature rise necessary forreading is as low as 200° C. or less, and the detection region in thelight spot is a high temperature portion relatively narrow in width inthe track direction, and therefore super-resolution reading action ispossible at a low working temperature, and therefore a read-onlymagneto-optical recording medium of high S/N ratio capable of narrowingthe track pitch is realized.

As evident from the principle of the invention, the material andthickness of the constituent elements are not limited to them alone,and, for example, the first and second protection layers may be composedof other nitride films, ZnS film or other chalcogenide films, SiO filmor other oxide films, or their mixture films, and the magnetic film maybe composed of other rare earth-transition metal compound magnetic filmshaving relatively high magneto-optical effects, such as TbFe, GdTbFe,TbFeCo, DyFe, GdDyFe, DyFeCo, GdDyFeCo, and NdTbFeCo.

That is, concerning the film composition, the presence of the magneticfilm differing in coercive force depending on the information to berecorded is the essential constituent element of the invention, whilethe protection layers, reflectance layer and protection coat layers areprovided properly only for keeping or improving the reliability, signalquality, or properties about heat distribution in recording and readingor the like.

In this magneto-optical recording medium, still more, corresponding tothe recording information, when the portion differing in the roughnessof surface of the substrate only by a specific value is provided in onlya partial region on the substrate (for example, the inner peripheralregion or the outer peripheral region), since the rare earth-transitionmetal compound magnetic film itself is a rewritable magnetic film, therewritable portion can be also formed as a magneto-optical recordingmedium (so-called partial ROM) disposed on the same plane.

In this embodiment, meanwhile, the recording domain area 31 wasroughened to be larger in coercive force than in the other area, butwhen the area other than the recording domain area 31 may be roughenedto be larger in coercive force than in the recording domain area 31, theobject of the invention of super-resolution reading action is possibleby the recording method mentioned in the Operation of the Invention.

Second embodiment

In the magneto-optical recording medium according to the secondembodiment of the invention in the medium constitution shown in FIG. 2,the magnetic film 23 is composed of noble metal/transition metalmulti-layer film, transition metal oxide and nitride compound film,ferrite film or other 3d transition metal compound magnetic filmpossessing a relatively high magneto-optical effect, and glass is usedas the substrate 21. The constituent elements of the magneto-opticalrecording medium of this embodiment may be same as in Embodiment 1,except for the magnetic film 23.

First, in the case of a noble metal/transition metal multi-layer filmsuch as Pt/Co and Pd/Co fabricated by a DC sputtering method on adielectric film such as SiN film, since the perpendicular magneticanisotropy energy Ku is about 0.7 to 2×10⁸ erg/cc, and the exchangestiffness constant A is about 0.8 to 1.3×10⁻⁸ erg/cm, the width δ ofthis domain wall is estimated to be around 20 to 43 nm. Herein, the filmthickness per layer of noble metal film is 0.8 to 3.5 nm, the filmthickness per layer of transition metal is 0.1 to 1.5 nm, and the entirefilm thickness of the noble metal/transition metal multi-layer film is15 to 40 nm.

In the case of a transition metal oxide and nitride compound film suchas FeON and FeCoON fabricated on a dielectric film such as SiN film by areactive ion beam sputtering method or the like, since the perpendicularmagnetic anisotropy energy Ku is about 4 to 8×10⁵ erg/cc, and theexchange stiffness constant A is about 0.6 to 1.2×10⁻⁸ erg/cm, the widthδ of this domain wall is estimated to be around 30 to 50 nm. Herein, thefilm thickness of the transition metal oxide and nitride compound filmwas 20 to 60 nm.

In the case of a ferrite film such as Co ferrite and Bi Co substitutiongarnet ferrite fabricated on a dielectric film such as SiN film by areactive sputtering method or the like, since the perpendicular magneticanisotropy energy Ku is about 0.2 to 1.5×10⁸ erg/cc, and the exchangestiffness constant A is about 0.3 to 1.5×10⁻⁸ erg/cm, the width δ ofthis domain wall is estimated to be around 30 to 50 nm. Herein, the filmthickness of the ferrite film was 100 to 300 nm.

Incidentally, in the schematic magnified view of the substrate 21 inFIG. 3, the same as in Embodiment 1, the roughness of surface in therecording domain area 31 is expressed in dimension α in the in-planedirection and dimension β in the perpendicular direction, and each meanin the recording domain area 31 is respectively <α> and <β>.

Such substrate 21 is fabricated, the sameness in Embodiment 1, by usinga master-substrate forming roughness portion and flat portion (smoothportion) corresponding to the recording information, easily byconventional mass copying methods of substrates such as the injectionmethod.

FIG. 10 shows the relation between the coercive force and the meanroughness <α> in the in-plane direction of the magnetic film (Pt/Comulti-layer film, FeCoON film, Bi Co substitution YIG film i.e., a YIGfilm in which a part of yttruim or iron is substituted with Bi or Co) 23with the mean roughness <β> in the perpendicular direction in therecording domain area 31 of up to 7 nm. FIG. 11 shows the relationbetween the coercive force and mean roughness <β> in the perpendiculardirection of the magnetic film 23 with <α> up to 45 nm.

It is known from FIG. 10 that the mean roughness <α> in the in-planedirection for starting to increase the coercive force is about 20 nm inthe Pt/Co multi-layer film, about 30 nm in the FeCoON film, and about 30nm in the Bi Co substitution YIG film. It is known from FIG. 11 that themean roughness <β> in the perpendicular direction for starting toincrease the coercive force is about 5 nm in all of the Pt/Comulti-layer film, FeCoON film and Bi Co substitution YIG film.

This result proves the action of the invention that the coercive forceis changed by a specific magnitude relation of the width of the domainwall and micro-structure as the micro-structure reflecting the roughnessof surface on the substrate is present as mentioned above.

When the temperature dependence of the coercive force was measured inindividual portions of the magneto-optical recording medium of theembodiment comprising the portion with <α> up to 45 nm and <β> up to 7nm and the portion with <α> up to 80 nm and <β> up to 1.5 nm on thesubstrate, a coercive force difference of about 1.6 times was confirmedin all temperature regions from room temperature to curie temperature.

Hence, by forming a magnetic film composed of a noble metal/transitionmetal multi-layer film such as Pt/Co multi-layer film, a transitionmetal oxide and nitride compound film such as FeCoON film, and a ferritefilm such as Bi Co substitution YIG film, depending on the recordinginformation, on the substrate comprising the portion with the meanroughness in the in-plane direction and in the perpendicular directionof 20 nm or more and 5 nm or more, respectively, and the portion withthe mean roughness in the in-plane direction and in the perpendiculardirection of 20 nm or less and 5 nm or less, respectively, amagneto-optical recording medium possessing portions differing incoercive force depending on the recording information is obtained.

However, same as in Embodiment 1, in order to achieve thesuper-resolution reading action of the invention by preventing noisegeneration and change of reflectivity in reading derived fromroughening, the mean roughness in the in-plane direction <α> must bedefined at 50 nm or less, and the mean roughness in the perpendiculardirection <β> at 20 nm or less.

Therefore, by reading this magneto-optical recording medium in thereading method mentioned in the Summary of the Invention same as inEmbodiment 1, super-resolution reading action is possible at a lowworking temperature, and the read-only magneto-optical recording mediumof high S/N ratio capable of narrowing the track pitch can be realized.

As evident from the principle of the invention, the material andthickness of the constituent elements are not limited to them alone,and, for example, the first and second protection layers may be composedof other nitride films, ZnS film or other chalcogenide films, SiO filmor other oxide films, or their mixture films, and in particular thesecond protection layer may be properly omitted. As for the magneticfilm, as far as the width δ of the domain wall is about 20 to 50 nm,other 3d transition metal magnetic film with a relatively highmagneto-optical effect may be used.

That is, concerning the film composition, the presence of the magneticfilm differing in coercive force depending on the information to berecorded is the essential constituent element of the invention, whilethe protection layers, reflectance layer and protection coat layers areprovided properly only for keeping or improving the reliability, signalquality, or properties about heat distribution in recording and readingor the like.

In this magneto-optical recording medium, still more, corresponding tothe recording information, when the portion differing in the roughnessof surface of the substrate only by a specific value is provided in onlya partial region on the substrate (for example, the inner peripheralregion or the outer peripheral region), since each above mentionedmagneto-optical magnetic film itself is a rewritable magnetic film, therewritable portion can be also formed as a magneto-optical recordingmedium (a so-called partial ROM) disposed on the same plane.

In this embodiment, meanwhile, the recording domain area 31 wasroughened to be larger in coercive force than in the other area, butwhen the area other than the recording domain area 31 may be roughenedto be larger in coercive force than in the recording domain area 31, theobject of the invention of super-resolution reading action is possibleby the recording method mentioned in the Operation of the Invention.

Third embodiment

FIG. 12 shows the constitution of the recording medium in a thirdembodiment of the invention. In FIGS. 12(a) and 12(b), reference numeral11 is a magneto-optical film composed of GdTbFeCo with the coerciveforce at room temperature of 900 Oe, curie temperature of 350° C. ormore, and compensation temperature of 0° C. or less. Thismagneto-optical film 11 is, as shown in FIG. 12(b), formed on asubstrate 21 through a first protection layer 22 made of a dielectricfilm. Herein, the substrate 21 is a polycarbonate substrate forming aV-shaped track guide groove in track pitch of 1.4 um, and the firstprotection layer 22 is a silicon nitride film with a thickness of 100nm. On the magneto-optical film 11, a second protection layer 24 made ofdielectric film, and a third protection layer 26 made of a resin layerare formed. Herein, the second protection layer 24 is a silicon nitridefilm with a thickness of 20 nm, and the third protection layer 26 is aurethane resin with a thickness of 5 um. Such film compositions areexactly the same as in the hitherto proposed compositions. What ischaracteristic of this embodiment is that a low coercive force portion11a and a high coercive force portion 11b corresponding to theinformation to be recorded are provided in the magneto-optical film 11as shown in FIGS. 12(a) and (b). In this embodiment, as shown in FIG.12(a), the portion corresponding to the written mark area is the lowcoercive force portion, and the portion corresponding to the other areais the high coercive force portion.

A method of fabricating a magneto-optical film differing in coerciveforce corresponding to the information to be recorded is mentionedbelow. The constitution of the apparatus for forming written marksdiffering in coercive force is basically the same as in the hithertoproposed recording apparatus.

Using this recording apparatus, light pulses were irradiated at variouspulse intervals, at a linear velocity of 1.3 m/s, pulse width of 100 nm,and peak power of 12 mW. This condition is the power of more than twotimes that of ordinary magneto-optical recording, and deterioration ofperpendicular magnetic anisotropy occurs in the beam irradiated portion,and the coercive force is lowered consequently. When the recordingmedium irradiated with a light pulse by such means was first uniformlymagnetized at 2 kOe and magnetized again at 500 Oe in a reversedirection, a reading signal was not obtained at all. However, by firstuniformly magnetizing at 2 kOe or more and then magnetizing again at 600Oe in reverse direction, when reproduced, a signal corresponding to therecorded signal was read. Therefore, in the condition of thisembodiment, it is known that the coercive force is lowered to about 600Oe or less in the beam irradiated portion. In this embodiment, byturning on or off the semiconductor laser according to the informationto be recorded, the low coercive force portion 11a was formed, but it isalso effective to turn on or off by EO modulator by using argon laser.

In particular, the forming method of written marks in this embodiment iscapable of forming the low coercive force portion 11a by an ordinarydisk drive, and therefore it is effective as a write-once disk, ratherthan as a read-only disk.

In the thus constituted magneto-optical recording medium, it was firstconfirmed that the recording magnetic domains can be formed along withelevation of temperature by means of a polarizing microscope. First theentire recording medium was magnetized at 1.2 kOe, and a magnetic fieldof 400 Oe was applied, and while heating the recording medium with asheathed heater, formation of magnetic domains was observed by thepolarizing microscope. As a result, no magnetic domain was observed atroom temperature, but when heated to 110° C. or more, recorded magneticdomains were observed, and when further heated over 190° C., themagnetic domains disappeared again. It means that, in the temperaturerange from 110° C. to 190° C., the magnetization is inverted in themagnetic field of 400 Oe only in the portion lowered in coercive forceby the light pulse irradiation for recording, and that, over 190° C.,the magnetization is inverted also in the portion not irradiated withthe light pulse. As known from these results, in this embodiment, whenH1 shown in FIG. 1 is 400 Oe, T1 is 110° C. and T2 is 190° C.

In the magneto-optical reading apparatus shown in FIG. 13, the samereading as in Embodiment 1 was conducted. The linear velocity was 5 m/s.What is particularly characteristic of the embodiment in reading themagneto-optical recording medium lies in the magnetic field applyingmeans 132, while the other constitution is the same as in the ordinarymagneto-optical reading apparatus hitherto proposed. The magnetic fieldapplying means 132 applies, as shown in FIG. 13, an initializingmagnetic field 13 from the S pole and a bias magnetic field 14 from theN pole to the magneto-optical film 11, and the bias magnetic field 14from the N pole is applied to the position of the optical beam forreading. Each magnetic field may be always constant in direction andintensity, and therefore the magnetic field applying means 132 ispreferred to be a permanent magnet from the viewpoint of saving of powerconsumption and reduction of size. Moreover, by forming it in a sizeenough for covering the recording region in the radial direction of thedisk, it is more effective because it is not necessary to move themagnetic field applying means 132. In the embodiment, the initializingmagnetic field 13 on the magneto-optical film surface is 1.2 kOe, andthe bias magnetic field 14 from the N pole is 400 Oe.

As a result of reading in this manner, the result nearly the same as inEmbodiment 1 shown in FIG. 9 was obtained.

Fourth embodiment

FIG. 14 is a sectional view showing the constitution of themagneto-optical recording medium in a fourth embodiment of theinvention, and FIGS. 15(a) and 15(b) schematically show the recordingapparatus for varying partially the magnetic anisotropy of the magneticfilm corresponding to the recording information in the magneto-opticalrecording medium of the embodiment.

In FIG. 14, a first protection layer 142, a magnetic film 143, anadditive element film 144, a second protection layer 145, and areflectance layer 146 are formed sequentially on a substrate 141 by a RFsputtering method and a DC sputtering method, and moreover a protectioncoat layer 147 is formed by spin coating method, and a magnetic film 128is composed of the magnetic film 143 and additive element film 144.Showing examples of materials of these constituent elements, thesubstrate 141 is made of polycarbonate, transparent plastic or glass,the first protection layer 142 and second protection layer 145 are madeof SiN or other nitride compound film, the magnetic film 143 is a rareearth-transition metal magnetic film such as GdTbFeCo film, the additiveelement film 144 is a heavy rare earth metal film such as Tb film, thereflectance layer 146 is a metal film such as Al film, and theprotection coat layer 147 is an epoxy UV resin. The thickness of eachfilm is, for example, 100 nm for the first protection layer 142, 15 to30 nm for the magnetic film 143, 0.1 to 0.4 nm for the additive elementfilm 144, 10 to 20 nm for the second protection layer 145, 40 nm for thereflectance layer 146, and 5 μm for the protection coat layer 147.

In FIG. 15(b), reference numeral 151 is a magneto-optical recordingmedium with already formed films on the substrate (films other than themagnetic film and additive element film are omitted for the sake ofconvenience), 141 is a substrate, 143 is a magnetic film, 143a is arecorded domain area, 144 is an additive element film, 152 is arecording laser beam, 153 is an optical head, and 154 is a spindlemotor.

By rotating the magneto-optical recording medium 151 at a constant speedof 1.4 m/sec, while with scanning the recording laser beam 152 with theoptical head 153, it is irradiated corresponding to the recordinginformation, at an intensity of more than 1.5 times the ordinaryrecording power of the rewritable magnetic film of GdTbFeCo film (inthis embodiment, in the composition of Gd₁₈ Tb₆ Fe₇₁ Co₅, at a curietemperature of about 230° C.). By this recording process, in therecording domain area 143a, the magnetic film GdTbFeCo film and additiveelement Tb film diffuse mutually to be formed into one, and only in thisarea the Tb content in the composition of the magnetic film 143increases.

As a result, corresponding to the recording information, the magneticanisotropy of the recording domain area 143a becomes larger, and thecoercive force in this area becomes larger than in the other area.

FIG. 16 shows the relation between the Tb amount and coercive force inthe GdTbFeCo film. When the composition is expressed by Tbx[Gd₁₉ Fe₇₆Co₅ ]_(100-x), when the Tb amount x increases from 6 at % to 6.5 at %,it is known that the coercive force is increased from 1 kOe to 1.8 kOe.

That is, by recording the magneto-optical recording medium having amagnetic film composed of a laminate film of magnetic film and anadditive element film as in this embodiment in the process as mentionedabove, the composition of the magnetic film is partially changed, andthe magneto-optical recording medium having portions differing in thecoercive force corresponding to the recording information can beobtained. Actually, in the thus obtained magneto-optical recordingmedium, when the temperature dependence of the coercive force wasmeasured in each portion, it was confirmed that the coercive forcedifference of about 1.5 times was obtained in all temperature regionsfrom room temperature to curie temperature.

Incidentally, the reflectivity of the magnetic film was hardly changedby slightly increasing the additive element, and the form of themagnetic film was hardly changed, and hence it was no cause forincreasing the reading noise.

Therefore, by reading this magneto-optical recording medium in thereading method mentioned in the Summary of the Invention in the samemanner as in Embodiment 1, super-resolution reading action is possibleat a low working temperature, and therefore a read-only magneto-opticalrecording medium of high S/N ratio capable of narrowing the track pitchis realized. Still more, the forming method of recording domains as inthis embodiment is applicable in an ordinary optical disk drive, andhence it is usable not only in a read-only disk but also in a write-oncedisk.

As evident from the principle of the invention, the material andthickness of the constituent elements are not limited to them alone,and, for example, the first and second protection layers may be composedof other nitride films, ZnS film or other chalcogenide films, SiO filmor other oxide films, or their mixture films, and the magnetic film maybe composed of other rare earth-transition metal compound magnetic filmshaving relatively high magneto-optical effects, such as TbFe, GdTbFe,TbFeCo, DyFe, GdDyFe, DyFeCo, GdDyFeCo, and NdTbFeCo, or a transitionmetal oxide and nitride compound film, a ferrite film, or a 3dtransition metal magnetic film, and the additive element film may be aheavy rare earth metal film aside from Tb.

That is, concerning the film composition, the presence of the magneticfilm differing in coercive force depending on the information to berecorded is the essential constituent element of the invention, whilethe protection layers, reflectance layer and protection coat layers areprovided properly only for keeping or improving the reliability, signalquality, or properties about heat distribution in recording and readingor the like.

In this magneto-optical recording medium, still more, corresponding tothe recording information, when the portion differing in the roughnessof surface of the substrate only by a specific value is provided in onlya partial region on the substrate (for example, the inner peripheralregion or the outer peripheral region), since the rare earth-transitionmetal compound magnetic film itself is a rewritable magnetic film, therewritable portion can be also formed as a magneto-optical recordingmedium (so-called partial ROM) disposed on the same plane.

In this embodiment, meanwhile, by varying the composition so that themagnetic anisotropy of the recording domain area 143a may be larger thanin the portion other than the recording domain area 143a, the recordingdomain area 143 is a portion of large coercive force, but to thecontrary, if the composition is changed so that the magnetic anisotropyof the recording domain area 143a may be smaller than in the portionother than the recording domain area 143a so that the recording domainarea 143a may be a portion of small coercive force, the object of theinvention of super-resolution reading action is possible by therecording method mentioned in the Operation of the Invention.

In this case, the magneto-optical recording medium is obtained byrecording in the recording apparatus in FIG. 15, by using GdTbFeCo filmas the magnetic film 143 in FIG. 14 and Fe film as the additive elementfilm 144. It is understood from the relation of the Fe amount andcoercive force in the GdTbFeCo film shown in FIG. 17, that is, whenexpressed in the composition of Fex [Gd₆₂ Tb₂₀ Co₁₈ ]_(100-x), thecoercive force is decreased from 1.7 kOe to 0.9 kOe when the Fe contentx is increased from 70.5 at % to 71.5 at %. Herein, the additive elementfilm may be a Cd or other 3d transition metal film.

Fifth embodiment

FIG. 18 is a sectional view showing the constitution of amagneto-optical recording medium in a fifth embodiment of the invention.

In FIG. 18, a first protection layer 182, a magnetic film 183, a secondprotection layer 184, and a reflectance layer 185 are sequentiallyformed on a substrate 181 by a RF sputtering method and a DC sputteringmethod or the like, and further a protection coat layer 186 is formed bya spin coating method. Showing examples of materials of constituentelements, the substrate 181 is made of glass, the first protection layer182 and second protection layer 184 are SiN or other nitride compoundfilm, the magnetic film 183 is a ferrite film such as a Bi Cosubstitution YIG film or a transition metal oxide and nitride compoundfilm such as FeCoON, the reflectance layer 185 is a metal film such asAl film, and the protection coat layer 186 is an epoxy UV resin. Thethickness of the film is, for example, 100 nm for the first protectionlayer 182, 100 to 300 nm for the magnetic film 183, 10 to 20 nm for thesecond protection layer 184, 40 nm for the reflectance layer 185, and 5μm for the protection coat layer 166.

In the magneto-optical recording medium of the embodiment, correspondingto the recording information, the micro-structure or crystal grain sizeobserved by transmission electron microscope in the magnetic film isvaried partially, and therefore the recording apparatus used in thefourth embodiment shown in FIG. 15 is used.

By rotating the magneto-optical recording medium 151 at a constant speedof 1.4 m/sec, while scanning the recording laser beam 152 with theoptical head 153, it is irradiated corresponding to the recording filmat an intensity of more than 2.0 times the ordinary recording power ofthe rewritable magnetic film of Bi Co substitution YIG film (in thisembodiment, in the composition of (Bi, Y)₃ Fe₃.4 (Co, Ge)₁.6 O₁₂, thecurie temperature is about 300° C.). By this recording process, in therecording domain area 143a, the Bi Co substitution YIG film is heated,and only in this area the crystal grains of the magnetic film are grown.

As a result, corresponding to the recording information, the mean sizeof crystal grain becomes larger in the recording domain area 143a thanin other area.

Meanwhile, if there is a micro-structure or crystal grain boundary dueto fluctuation in composition, density, crystallinity or the like whichcan be observed by the transmission electron microscope in the magneticfilm, the homogeneity of the film is lost in that portion, and adisturbance occurs in the domain wall energy. Accordingly, since thedomain wall energy is the energy reserved in the domain wall, if thedomain wall energy varies with the position, in order to move the domainwall, the force expressed by its differential value is required. On theother hand, the domain wall is the region of gradual transition of thedirection of spin as a source of magnetization, and possesses a specificwidth, and therefore when the micro-structure or crystal grain sizebecomes smaller than the width of the domain wall, the heterogeneityderived from them in the domain wall is averaged, and the interaction ofthe domain wall with them becomes smaller, and the movement of thedomain wall becomes easy. That is, it is possible to change the coerciveforce by a certain magnitude relation between the width of the domainwall and the micro-structure or crystal grain size.

In the case of a ferrite film such as a Bi Co substitution garnetferrite fabricated by a reactive RF sputtering method on a dielectricfilm such as SiN film, since the perpendicular magnetic anisotropyenergy Ku is about 0.2 to 1.5×10⁸ erg/cc, and the exchange stiffnessconstant A is about 0.3 to 1.5×10⁻⁸ erg/cm, the width δ of its domainwall is estimated to be around 30 to 50 nm.

In the case of a transition metal oxide and nitride compound film suchas FeON and FeCoON fabricated by a reactive ion beam sputtering methodon a dielectric film such as a SiN film, since the perpendicularmagnetic anisotropy energy Ku is about 4 to 8×108 erg/cc, and theexchange stiffness constant A is about 0.6 to 1.2×10-8 erg/cm, the widthd of its domain wall is estimated to be around 30 to 50 nm.

Therefore, by properly adjusting the substrate temperature whendepositing the magnetic film, a magneto-optical recording medium withthe size of the micro-structure or crystal grain in the magnetic film of30 nm or less is fabricated, and then by recording with the use of therecording apparatus shown in FIG. 15(b), the micro-structure or crystalgrain in the recording domain area 143a is grown over 30 nm which is thedomain wall width of the magnetic film, so that the coercive force inthe recording domain area 143a becomes larger than in the other area.

In the magneto-optical recording medium of the embodiment growing thecrystal grain to an average size of about 45 nm, by forming a Bi Cosubstitution YIG film with a mean crystal grain size of about 15 nm onthe substrate and heating partly, as a result of investigation oftemperature dependence of coercive force in each part, it was confirmedthat a coercive force difference of about 1.5 times was obtained in alltemperature regions from room temperature to curie temperature.

Thus, by forming the magnetic film having the portion of 30 nm or moreand the portion of 30 nm or more in the mean size of the micro-structureor crystal grain in the film, corresponding to the recordinginformation, the magneto-optical recording medium having portionsdiffering in the coercive force depending on the recording informationis obtained.

However, in order to increase the coercive force, if the mean size ofthe micro-structure or crystal grain in the magnetic film is more than30 nm, it is not enough for the magneto-optical recording medium of theembodiment. That is, it is necessary to prevent generation of noise atthe time of reading derived from expansion of the average size ofmicro-structure or crystal grain in the magnetic film. To suppress thenoise, the mean size of the micro-structure or crystal grain in themagnetic film should be 50 nm or less.

Hence, the mean size of the micro-structure or crystal grain in themagnetic film in the high coercive force portion is desired to be 30 to50 nm.

Therefore, by reading this magneto-optical recording medium in thereading method mentioned in the Summary of the Invention in the samemanner as in Embodiment 1, super-resolution reading action is possibleat a low working temperature, and therefore a read-only magneto-opticalrecording medium of high S/N ratio capable of narrowing the track pitchis realized. Still more, the forming method of recording domains as inthis embodiment is applicable in an ordinary optical disk drive, andhence it is usable not only in a read-only disk but also in a write-oncedisk.

As evident from the principle of the invention, the material andthickness of the constituent elements are not limited to them alone,and, for example, the first and second protection layers may be composedof other nitride films, ZnS film or other chalcogenide films, SiO filmor other oxide films, or their mixture films, and the magnetic film maybe composed of other 3d transition metal magnetic films having arelatively high magneto-optical effect with the width δ of about 20 to50 nm (Co ferrite film, other ferrite film, FeON, FeCoON, or othertransition metal oxide and nitride compound films).

That is, concerning the films composition, the presence of the magneticfilm differing in coercive force depending on the information to berecorded is the essential constituent element of the invention, whilethe protection layers, reflectance layer and protection coat layers areprovided properly only for keeping or improving the reliability, signalquality, or properties about heat distribution in recording and readingor the like.

In this magneto-optical recording medium, still more, corresponding tothe recording information, when the portion differing in the roughnessof surface of the substrate only by a specific value is provided in onlya partial region on the substrate (for example, the inner peripheralregion or the outer peripheral region), since the 3d transition metalcompound magnetic film itself is a rewritable magnetic film, therewritable portion can be also formed as a magneto-optical recordingmedium (a so-called partial ROM) disposed on the same plane.

Sixth embodiment

Another constituent example of a recording medium according to theinvention is described below.

FIG. 19 shows a reading principle in an application example of theinvention. In this application example, a magnetic film is formedthrough a protection layer 192 on a substrate 191 comprising flatportions and roughness portions depending on the information to berecorded. The magnetic film comprises a first magnetic layer 193, asecond magnetic layer 194, and a third magnetic layer 195.

The third magnetic layer 194 differs in the coercive force between flatportion 195a and roughness portion 195b, and after once magnetizing inbatch in a magnetic field greater than either coercive force, bymagnetizing again in a magnetic field in a reverse direction larger thanone coercive force and smaller than the other coercive force, a writtenmark is formed. For the third magnetic layer 195, a film thickness ofabout 40 to 60 nm is appropriate, in a rare earth-transition metalmagnetic film such as TbFeCo film.

The first magnetic layer 193 possesses the characteristics to be anin-plane magnetized film at room temperature and a perpendicularmagnetized film at a high temperature (100 to 300° C.) as shown in FIG.20. The magnetic film having such characteristics is realized bypreparing the composition of rare earth-transition metal alloy havingthe characteristics to be a perpendicular magnetized film only near thecompensation composition, and to vary in the compensation composition tohave two types of spin in a balanced state with the temperature. Aboveall, the GdFeCo alloy is likely to realize the characteristics offorming an in-plane magnetized film at room temperature and aperpendicular magnetized film at high temperature (100 to 300° C.), andin the example of GdFeCo, the composition of about 26 to 31% of Gd, 50to 70% of Fe, and 5 to 10% of Co is preferred, and Gd₂₉ Fe₆₁ Co₁₀ is oneof the most preferable compositions.

The second magnetic layer 194 is to adjust the exchange coupling forceacting between the first magnetic layer and the third magnetic layer,and provides the following two functions. One is the role of forming adomain wall stably between the first magnetic layer and the thirdmagnetic layer when the first magnetic layer 194 is an in-planemagnetized film, and the other is the function of cutting off theexchange coupling force acting between the first magnetic layer and thethird magnetic layer as the magnetism is lost above the curietemperature. As the second magnetic layer 194, the magnetic film in thein-plane anisotropy at curie temperature of 200 to 250° C. is preferablyin a thickness of about 3 to 20 nm. These may be realized by the rareearth-transition metal film such as GdFeCo.

The third magnetic layer 195 is provided with, if necessary, aprotection layer (not shown), and the reading actions are explainedbelow.

By two times of magnetization differing in intensity and direction, astatus of magnetization mutually opposite in the flat portion androughness portion depending on the information to be recorded can beformed in the third magnetic layer, and a written mark is formed. Bycontrast, the first magnetic layer is an in-plane magnetized filmregardless of the flat portion or roughness portion, and when exposed toan optical beam for reading 12, the magnetic film is raised intemperature. When the temperature exceeds T3, the first magnetic layeris a perpendicular magnetized film. This temperature, that is, thetemperature for changing the anisotropy from the in-plane to theperpendicular direction is hardly different in the flat portion androughness portion. At this time, the coercive force of the firstmagnetic layer is 100 Oe or less, and by exchange coupling, the writtenmark recorded in the third magnetic layer is easily copied into thefirst magnetic layer. When the temperature rises further until exceedingthe curie temperature T4 of the second magnetic layer 194, themagnetization of the second magnetic layer is lost, thereby cutting offthe exchange coupling acting between the first magnetic layer 193 andthe third magnetic layer 195. At this time, the magnetization of thefirst magnetic layer 193 has a coercive force of 100 Oe or less whetherin the flat portion or in the roughness portion, and the application ismade in one direction regardless of the flat portion or roughnessportion by the action of the external magnetic field 14.

Therefore, the reading signal is obtained only in the range between thetemperature T1 and the temperature T2 in the reading beam irradiationregion.

Thus, by contrast to the medium composition of rewritablesuper-resolution proposed hitherto, by forming a magnetic layercomprising different coercive force corresponding to the information tobe recorded, a read-only medium of super-resolution is realized. Thisholds true with other rewritable compositions of super-resolution.

In this magneto-optical recording medium, moreover, by disposing theportion differing in the roughness of surface of substrate only by acertain value corresponding to the recording information, only in apartial region on the substrate (for example, the inner peripheralregion or the outer peripheral region), an optical reading mediumdisposing a read-only super-resolution region and a rewritablesuper-resolution region on a same plane (a so-called super-resolutionpartial ROM) may be realized.

Seventh embodiment

Another constituent example of a recording medium according to theinvention is described.

FIGS. 21(a) and 21(b) show a different principle of reading in anapplication of the invention. In this application example, on asubstrate 211 forming flat portions and roughness portions depending onthe information to be recorded, a magnetic film 213 is disposed througha protection layer 212. The magnetic film 213 is a magnetic film havinga compensation temperature T6, and should be preferably GdFeCo having aperpendicular magnetic anisotropy energy only in the vicinity of thecompensation temperature. The magnetic film 213 comprises a flat portion213a and a roughness portion 213b, and both are in-plane magnetizedfilms at room temperature, and there is no difference in coercive force.However, they are transformed into perpendicular magnetized films attemperature T5 higher than room temperature. The coercive force of themagnetic film at temperature T5 is smaller than that of the magneticfield H1 given in the reading magnetic field 14, and when transformedinto perpendicular magnetized films, simultaneously, they are magnetizedin the direction of H1, whether flat portion 213a or roughness portion213b. However, the magnetic film 213 is changed from rare earth dominantto transition metal dominant magnetically at temperature T6. At thistime, the coercive force of the magnetic film 213 is extremely large,and it is not affected by the reading magnetic field H1. However, whenthe temperature further rises, the coercive force decreases again, andthe coercive force difference becomes large between the flat portion213a and roughness portion 213b. When reaching temperature T7, thecoercive force of the flat portion 213a becomes smaller than H1, and themagnetization is inverted. At this time, since the coercive force of theroughness portion 213b remains higher than H1, the magnetization is notinverted.

When the temperature further rises to exceed T8, the coercive force ofthe roughness portion 213b becomes smaller than H1, and themagnetization is inverted. That is, only in the portion of T7 or moreand T8 or less, the information is read. After passing of the opticalbeam for reading, by natural cooling, it is transformed into thein-plane magnetized film again, and it is not necessary to initialize atthis time.

As explained herein, in the invention, if it is an in-plane magnetizedfilm regardless of the information to be recorded at room temperature,super-resolution reading is possible by providing a coercive forcedifference depending on the information to be recorded at a temperatureabove room temperature (T7 or more and T8 or less in this embodiment).

In this magneto-optical recording medium, moreover, by disposing theportion differing in the roughness of surface of substrate only by acertain value corresponding to the recording information, only in apartial region on the substrate (for example, inner peripheral region orouter peripheral region), an optical reading medium disposing aread-only super-resolution region and a rewritable super-resolutionregion on a same plane (a so-called super-resolution partial ROM) may berealized.

What is claimed is:
 1. A magnetic recording medium, being amagneto-optical recording medium possessing a read-only portion and arewritable portion, wherein the read-only portion is composed of amagnetic film possessing different magnetic anisotropy depending oninformations to be recorded, and the rewritable portion is composed of amagnetic film possessing a uniform magnetic characteristic regardless ofthe informations to be recorded.
 2. A magnetic recording medium, being amagneto-optical recording medium possessing a write-once portion and arewritable portion, wherein the magnetic anisotropy in a written markpart is different from the other part in the write-once portion, and therewritable portion is composed of a magnetic film possessing a uniformmagnetic characteristic regardless of the presence or absence of writtenmark.
 3. A manufacturing process of magnetic recording medium comprisinga step of applying a photo resist on a glass master-substrate andheating, a step of cutting the photo resist by a laser modulated inpower depending on recording information, a step of developing in adeveloping solution after cutting, a step of roughening the exposedportion of the glass master-substrate by plasma ion etching in using amixed gas of CF₄ gas and Ne gas or a mixed gas of CF₄ gas and Ar gas,form the photo resist surface side after developing, a step of removingall of the photo resist formed on the glass master-substrate afterplasma ion etching, a step of forming a stamper by electroforming on theglass master-substrate after removing the photo resist, a step ofpeeling off the stamper from the glass master-substrate, a step offorming a substrate possessing portions different in the roughness ofsurface corresponding to the recording information by using the stamper,and a step of depositing a magnetic film on the substrate.
 4. Amanufacturing method of magnetic recording medium comprising a step ofapplying a photo resist on a glass master-substrate and heating, a stepof roughening the photo resist surface by plasma ion etching in a mixedgas of N₂ gas and He gas or a mixed gas of O₂ gas and He gas, a step ofsmoothing by softening the roughened photo resist surface by lasermodulated in power depending on the recording information after plasmaion etching, a step of forming a stamper by electroforming on the photoresist after cutting, a step of peeling off the stamper from the glassmaster-substrate and photo resist, a step of forming a substratepossessing portions different in the roughness of surface depending onthe recording information by using the stamper, and a step of depositinga magnetic film on the substrate.
 5. A recording method of magneticrecording medium, wherein by recording at an intensity of light capableof mutually diffusing a multi-layered magnetic film and additive elementfilm, when recording information into a magneto-optical recording mediumcomposed of a magnetic film possessing a certain magnetic anisotropy anda film of an additive element for increasing or decreasing the magneticanisotropy of the magnetic film, such portions are formed that aredifferent in a magnitude of magnetic anisotropy, corresponding to therecording information, and are different in a coercive force of themagnetic film in each portion.
 6. A recording method of magneticrecording medium of claim 5, wherein the magnetic film to be layered isa rare earth-transition metal magnetic film, and the film of additiveelement is heavy rare earth metal film or transition metal film.
 7. Arecording method of magnetic recording medium, wherein by recording atan intensity of light capable of growing crystal grains ormicro-structures observed by transmission electron microscope in amagnetic film, when recording information into a magneto-opticalrecording medium of which magnetic film is composed of a crystallinemagnetic film, such portions are formed that are different in a size ofmicro-structures or crystal grains, depending on the recordinginformation, and are different in a coercive force of the magnetic filmin each portion.
 8. A reading method of magneto-optical recording mediumwherein varying a status of magnetization of a magneto-optical recordingmedium by a bias magnetic field, while reading a magneto-opticalrecording medium providing magneto-optical magnetic films possessingdifferent coercive forces depending on informations to be recorded, atleast in a part.
 9. A reading method of magneto-optical recording mediumof claim 8, wherein it is read while returning the changed status ofmagnetization into an initial status, by an initializing magnetic fieldprovided separately from the bias magnetic field.